Ryan Scott authored Jul 07, 2019 and Marge Bot committed Jul 11, 2019

Previously, GHC would typecheck the `via` type once per class in a
`deriving` clause, which caused the problems observed in #16923.
This patch restructures some of the functionality in `TcDeriv` and
`TcHsType` to avoid this problem. We now typecheck the `via` type
exactly once per `deriving` clause and *then* typecheck all of the
classes in the clause.
See `Note [Don't typecheck too much in DerivingVia]` in `TcDeriv`
for the full details.